Apparently you need only 23 people to have a 50% chance of having two or more in the group share the same birthday; this despite there being 365/6 days in the year and — presumably — birthdays being more or less evenly spread throughout the year (although, in fairness, this last might be an unwarranted assumption).
And, apparently, if you have a group of 30 people (randomly selected) there’s a 70% chance that two or more of them will share birth-dates!
To put this to the test BBC Radio 4’s ‘More or Less’ programme-makers ran a check on the squads at the World Cup (FIFA World Cup squads being made up of 23). And indeed, exactly as one would expect (statistically speaking), out of the 32 World Cup squads, 16 of them had two or more footballers sharing the same birthdays.
The Wordkern Archive 